You don't have to play multiplayer to get the best ending. I didn't play MP and still got
the synthesis option
. Granted, I pretty much did every side mission I could.
Wow, what a terrible ending. No matter what option you choose, the endings are the same and there's the one stupid epilogue.
Major endgame spoiler:
Seriously, the Citadel is the AI? Why didn't it help Sovereign in ME1? Why did it let the Keepers get reprogrammed?
Was that Bioware's plan the whole time, or did someone get high one evening and came up with that. :-/
I give the game, before the final mission, a 5/5. I think it's pretty sweet. As far as I'm concerned, the ending doesn't exist.